Impact of Water on PEFC Performance Evaluated by Neutron Imaging Combined with Pulsed Helox Operation

Abstract

A novel experimental method for the measurement of mass transport losses in a polymer electrolyte fuel cell (PEFC) was developed, based on the comparison of cell voltage during operation with air, helox, (79% He, 21% O 2 ) or pure O 2 . Using only short periods (2 seconds) of helox or pure O 2 operation, the disturbing artifacts (dry out or change of catalyst surface oxidation state) associated with continuous operation with these gases were avoided. High resolution neutron imaging was conducted simultaneously in order to study the relation between mass transport loss and liquid water accumulation. For the studied design, comparisons in steady state and dynamic experiments indicated a larger impact of water accumulation in the gas diffusion layers (GDL) under the flow channels, or in the portion of the channels near the GDL surface. Increased diffusion losses were observed, as expected, under high humidity conditions, but also at very low humidity conditions in absence of any liquid water. The latter was explained by a redistribution of current toward the rib region, having longer diffusion paths.Polymer Electrolyte Fuel Cells (PEFCs) are energy converters, bearing attractive characteristics for automotive applications in terms of efficiency and absence of pollutant emissions. The dimension of an automobile fuel cell system is primarily determined by the required peak power. In consequence, reaching the highest possible power density allows a reduction of the system size, weight, and cost. At high current density operation, losses introduced by the limitations in the diffusive transport of oxygen through the gas diffusion layers (GDLs) can become significant. The presence of liquid water in the gas diffusion layers reduces the effective diffusivity of such media and results in increased losses.The visualization of liquid water in operating PEFCs using different methods has been increasingly reported. A recent review on in situ visualization methods can be found in a publication by Tsushima et al. 1 Besides its high contrast for liquid water, neutron imaging 2-20 offers the advantages of good transparency of usual fuel cell materials (including metals), and of a negligible impact of the non-ionizing radiation on cell operation, making it an excellent non-invasive method. Water visualization in fuel cells has been a strong driving force for addressing the issue of the limited spatial resolution of neutron imaging. [19][20][21][22][23] The approach used at the Paul Scherrer Institut, based on optimized optical setup and scintillator screens, 24 combined with specific improvements for fuel cell imaging, 20,21 allowed reaching the combination of an effective spatial resolution of 20 μm with exposure times of 10 seconds. Thus, the measurement of liquid water content in different layers of a fuel cell is possible, not only in steady state, but also in transient operation. In order to assess the impact of liquid water on cell performance, a method for directly measuring diffusive transport losses is of high interest. For...

The Hydro-electro-thermal Performance of Air-cooled, Open-cathode Polymer Electrolyte Fuel Cells: Combined Localised Current Density, Temperature and Water Mapping

“…This exposure time is well within the range typically used for neutron imaging on PEFCs (typically between 1 and 25 s [45,[51][52][53][54][55][56][57][58][59]82]). Since the current study investigates steady-state operation, the 12 s temporal resolution is sufficient.…”

“…This feature is also in agreement with the measurements performed under the ribs and channels in closed-cathode configuration, with a dry cathode. The water almost exclusively concentrated under the land, and not under the channels [82,87].…”

Communication—Neutron Radiography of the Water/Gas Distribution in the Porous Layers of an Operating Electrolyser

“…For this purpose, we applied the anisotropic imaging technique which was established for fuel cells, [7][8][9][10][11][12][13] including the tilted-detector setup, 8 adapted for application to a small scale PEM electrolysis cell. * Electrochemical Society Member.…”

Развитие Метода Измерения Диффузионного Сопротивления Структур Материалов Пористых Электродов На Основе Протонпроводящего Иономера И Углеродных Наноматериалов

“…Наибольшее распространение при экспериментальном исследовании массотранспортных потерь получили два подхода, основанные на сравнении напряжения элемента во время работы с воздушно-гелиевой средой (79% He, 21% O 2 ) или чистым О 2 [11][12][13], и метод, основанный на измерении плотности предельного диффузионного тока [2,14,15]. В первом подходе использование корот-ких промежутков времени (2 s) позволяет исключить влияние высыхания или изменение степени окисле-ния поверхности катализатора, связанные с непрерыв-ной работой с этими газами, однако получаемые при использовании этого метода результаты не отражают диффузионные свойства в процессе функционирования электрода.…”